In recent years, the technical development of separation membranes has progressed, and owing to such features as space saving, labor saving and higher filtered water quality, the use of separation membranes is expanding in various fields including water treatment. For example, precision filtration membranes and ultrafiltration membranes are applied to the water purification processes for producing industrial water and tap water from river water, groundwater and sewage treatment water, and applied to the pretreatment in the reverse osmosis membrane treatment processes for seawater desalination and to membrane separation activated sludge processes. Nanofiltration membranes and reverse osmosis membranes are applied to ion removal, seawater desalination and wastewater reutilization processes.
In the recent situation where water shortage is acute and chronic, seawater desalination using nanofiltration membranes and reverse osmosis membranes is actively pursued. In the nanofiltration membrane/reverse osmosis membrane filtration method requiring a supply pressure higher than the osmotic pressure, a pump called “a booster pump” must be used for pressurization when raw water is supplied to nanofiltration membranes/reverse osmosis membranes. That is, if the salt concentration of raw water supplied to nanofiltration membranes/reverse osmosis membranes is higher, the osmotic pressure is higher, and therefore it is necessary to produce a higher pressure using a booster pump, and the energy for operating the booster pump is necessary.
In order to solve these problems, for example, membrane process systems in which advanced sewage treatment and seawater desalination are integrated are developed (non-patent documents 1 and 2). According to these technologies, the treatment of sewage by a membrane bioreactor is followed by the production of fresh water using reverse osmosis membranes, and further the concentrated water produced as a byproduct at the time of separation by the reverse osmosis membranes is joined with seawater. Therefore, the salt concentration of supplied seawater declines, and the pressurization by the booster pump for enforcing the reverse osmosis membrane separation for seawater desalination can be reduced. Thus, more energy-saving systems can be established.
Meanwhile, in a fresh water production system using semipermeable membranes, in general, the deposition of organic matter and turbid matter, the scale of metallic ions, the formation of the biofilm by microbial growth and the like can cause blocking on the surfaces of the semipermeable membranes and in the semipermeable membrane treatment apparatus, to bring about such troubles as the decrease in the quantity of produced fresh water and the rise of pressure. In particular, the formation of the biofilm is mainly caused by the microbes and substrate (carbon sources and nutrient salts) derived from the raw water, and the microbial growth occurs not only on the surfaces of the semipermeable membranes and in the semipermeable membrane treatment apparatus but also in the upstream piping. Accordingly troubles occur often. If the biofilm formed in a piping intermittently peels, the blocking of the channel such as the piping and tank and the abovementioned troubles of the semipermeable membrane treatment apparatus are promoted. In order to prevent such troubles, it is necessary to sterilize the semipermeable membrane treatment apparatus and the piping. Further, in the case where raw water with much organic matter such as sewage is treated by semipermeable membranes, the organic matter is deposited on the membrane surfaces, and microbes are likely to grow on the basis of the deposited organic matter. Therefore, it is necessary to wash the semipermeable membrane treatment apparatus using a chemical, for removing the organic matter.
In the case of a fresh water production system in which advanced sewage treatment and seawater desalination are integrated as described in non-patent document 1 or 2, the water obtained by treating sewage by a membrane bioreactor is treated as raw water by reverse osmosis membranes, to obtain fresh water, and the water concentrated by the reverse osmosis membranes, which is usually disposed as waste, is mixed with seawater, for further treatment by reverse osmosis membranes. In the concentrated water, the carbon sources and nutrient salts as the substrate of microbes are more concentrated than those of the biologically treated water, and consequently provide an environment where microbes are likely to grow. Thus, there is a problem that since a microbial film is formed in a concentrated water piping, the latter reverse osmosis membranes suffer troubles. Further, the aforementioned fresh water production system in which advanced sewage treatment and seawater desalination are integrated has an advantage that since the chemical used for the former reverse osmosis membranes is fed via the concentrated water piping also into the latter reverse osmosis membranes, the chemical used in the former reverse osmosis membranes can be reused also in the concentrated water piping and the latter reverse osmosis membranes. However, the chemical reused may decline in the washing/sterilization effect as the case may be, and the former reverse osmosis membranes, the concentrated water piping and the latter osmosis membranes may require respectively different optimum chemicals as the case may be. Therefore, the system has a problem that the washing/sterilization effects in the concentrated water piping and the latter reverse osmosis membranes are insufficient.
Further, it is known that a chemical such as sodium hypochlorite is added immediately after water intake also on the seawater side, but the chemical is, for example, consumed for sterilization of piping or diluted by joining/mixing with the concentrated water of the reverse osmosis membranes on the sewage side, to make the subsequent washing/sterilization effect insufficient. Furthermore, a biofilm can be formed in the mixed water piping, to block the mixed water piping, the reverse osmosis membrane treatment apparatus or the safety filter thereof. Moreover, after seawater is joined/mixed with the concentrated water of the reverse osmosis membranes on the sewage side, both the chemicals are mixed with each other or with a neutralizing agent, to pose such a problem that the washing/sterilization effect is decreased or that a harmful gas is generated. Further, in the case where a chemical is injected into the water supplied to the latter reverse osmosis membrane treatment apparatus, there is such a problem that if a chemical is newly injected though a chemical of the same type has been supplied on the upstream side, the injected amount of the chemicals of the same type becomes excessive, or that the chemical or neutralizing agent supplied on the upstream side decreases the effect of the newly injected chemical.
Further, as described in patent document 1, known is a method in which at least one chemical is added to the water supplied to reverse osmosis membranes and to the water concentrated by the reverse osmosis membranes, and the concentrated water is circulated into the water supplied, for reusing the chemical. However, this method is not applicable to a system for treating different types of raw water, and there has been no conventional method for effectively using chemicals to allow reliable washing/sterilization of piping, tanks and the like in a fresh water production system using a composite water treatment technology.